1. Field
The present invention relates to communications in a wireline or a wireless communication system. More particularly, the present invention relates to a method and system for a data transmission in such a communication system.
2. Background
Communication systems have been developed to allow transmission of information signals from an origination station to a physically distinct destination station. In transmitting an information signal from the origination station over a communication channel, the information signal is first converted into a form suitable for efficient transmission over the communication channel. Conversion, or modulation, of the information signal involves varying a parameter of a carrier wave in accordance with the information signal in such a way that the spectrum of the resulting modulated carrier wave is confined within the communication channel bandwidth. At the destination station, the original information signal is reconstructed from the modulated carrier wave received over the communication channel. In general, such a reconstruction is achieved by using an inverse of the modulation process employed by the origination station.
Modulation also facilitates multiple-access, i.e., simultaneous transmission and/or reception, of several signals over a common communication channel. Multiple-access communication systems often include a plurality of remote subscriber units requiring intermittent access of relatively short duration rather than continuous access to the common communication channel. Several multiple-access techniques are known in the art, such as time division multiple-access (TDMA), and frequency division multiple-access (FDMA). Another type of a multiple-access technique is a code-division multiple-access (CDMA) spread spectrum system that conforms to the “TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wide-Band Spread Spectrum Cellular System,” hereinafter referred to as the IS-95 standard. The use of CDMA techniques in a multiple-access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE-ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the present assignee.
A multiple-access communication system may be wireless or wire-line and may carry voice traffic and/or data traffic. An example of a communication system carrying both voice and data traffic is a system in accordance with the IS-95 standard, which specifies transmitting voice and data traffic over a communication channel. A method for transmitting data in code channel frames of fixed size is described in detail in U.S. Pat. No. 5,504,773, entitled “METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION,” assigned to the present assignee. In accordance with the IS-95 standard, the data traffic or voice traffic is partitioned into code channel frames that are 20 milliseconds wide with data rates as high as 14.4 Kbps. Additional examples of communication systems carrying both voice and data traffic comprise communication systems conforming to the “3rd Generation Partnership Project” (3GPP), embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), or “TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems” (the IS-2000 standard).
The term base station is an access network entity, with which subscriber stations communicate. With reference to the IS-856 standard, the base station is also referred to as an access point. Cell refers to the base station or a geographic coverage area served by a base station, depending on the context in which the term is used. A sector is a partition of a base station, serving a partition of a geographic area served by the base station.
The term “subscriber station” is used herein to mean the entity with which an access network communicates. With reference to the IS-856 standard, the subscriber station is also referred to as an access terminal. A subscriber station may be mobile or stationary. A subscriber station may be any data device that communicates through a wireless channel or through a wired channel, for example fiber optic or coaxial cables. A subscriber station may further be any of a number of types of devices including but not limited to PC card, compact flash, external or internal modem, or wireless or wireline phone. A subscriber station that is in the process of establishing an active traffic channel connection with a base station is said to be in a connection setup state. A subscriber station that has established an active traffic channel connection with a base station is called an active subscriber station, and is said to be in a traffic state.
The term access network is a collection of at least one base station (BS) and one or more base stations' controllers. The access network transports information signals between multiple subscriber stations. The access network may be further connected to additional networks outside the access network, such as a corporate intranet or the Internet, and may transport information signals between each base station and such outside networks.
In the above-described multiple-access wireless communication system, communications between users are conducted through one or more base stations. The term user refers to both animate and inanimate entities. A first user on one wireless subscriber station communicates to a second user on a second wireless subscriber station by conveying information signal on a reverse link to a base station. The base station receives the information signal and conveys the information signal on a forward link to the second subscriber station. If the second subscriber station is not in the area served by the base station, the base station routes the data to another base station, in whose service area the second subscriber station is located. The second base station then conveys the information signal on a forward link to the second subscriber station. The forward link refers to transmissions from a base station to a wireless subscriber station, and the reverse link refers to transmissions from a wireless subscriber station to a base station. Likewise, the communication can be conducted between a first user on a wireless subscriber station and a second user on a landline station. A base station receives the data from the first user on the wireless subscriber station on a reverse link, and routes the data through a public switched telephone network (PSTN) to the second user on a landline station. In many communication systems, e.g., IS-95, W-CDMA, and IS-2000, the forward link and the reverse link are allocated separate frequencies.
Study of voice traffic only services and data traffic only services revealed some substantial differences between the two types of services. One difference concerns delay in delivery of the information content. The voice traffic services impose stringent and fixed delay requirements. Typically, an overall one-way delay of a predetermined amount of voice traffic information, referred to as a speech frame, must be less than 100 ms. In contrast, the overall one-way data traffic delay may be a variable parameter, used to optimize the efficiency of the data traffic services provided by the communication system. For example, multi-user diversity, delay of data transmission until more favorable conditions, more efficient error correcting coding techniques, which require significantly larger delays than delays that can be tolerated by voice traffic services, and other techniques can be utilized. An exemplary efficient coding scheme for data is disclosed in U.S. patent application Ser. No. 08/743,688, entitled “SOFT DECISION OUTPUT DECODER FOR DECODING CONVOLUTIONALLY ENCODED CODEWORDS,” filed Nov. 6, 1996, now U.S. Pat. No. 5,933,462, issued Aug. 3, 1999, assigned to the present assignee.
Another significant difference between voice traffic services and data traffic services is that the former require a fixed and common grade of service (GOS) for all users. Typically, for digital communication systems providing voice traffic services, this requirement translates into a fixed and equal transmission rate for all users and a maximum tolerable value for the error rates of speech frames. In contrast, the GOS for data services may be different from user to user, and may be a variable parameter, whose optimization increases the overall efficiency of the data traffic service providing communication system. The GOS of a data traffic service providing communication system is typically defined as the total delay incurred in the transfer of a predetermined amount of data traffic information which may comprise, e.g., a data packet. The term packet is a group of bits, including data (payload) and control elements, arranged into a specific format. The control elements comprise, e.g., a preamble, a quality metric, and others known to one skilled in the art. Quality metric comprises, e.g., a cyclic redundancy check (CRC), a parity bit, and others known to one skilled in the art.
Yet, another significant difference between voice traffic services and data traffic services is that the former requires a reliable communication link. When a subscriber station, communicating voice traffic with a first base station, moves to the edge of the cell served by the first base station, the subscriber station enters a region of overlap with another cell served by a second base station. The subscriber station in such a region establishes a voice traffic communication with the second base station while maintaining a voice traffic communication with the first base station. During such a simultaneous communication, the subscriber station receives a signal carrying identical information from two base stations. Likewise, both of the base stations also receive signals carrying information from the subscriber station.
Such a simultaneous communication is termed soft handoff. When the subscriber station eventually leaves the cell served by the first base station, and breaks the voice traffic communication with the first base station, the subscriber station continues the voice traffic communication with the second base station. Because soft handoff is a “make before break” mechanism, the soft handoff minimizes the probability of dropped calls. A method and system for providing a communication with a subscriber station through more than one base station during the soft handoff process are disclosed in U.S. Pat. No. 5,267,261, entitled “MOBILE ASSISTED SOFT HAND-OFF IN A CDMA CELLULAR TELEPHONE SYSTEM,” assigned to the present assignee.
Softer handoff is a similar process whereby the communication occurs over at least two sectors of a multi-sector base station. The process of softer handoff is described in detail in U.S. patent application Ser. No. 08/763,498, entitled “METHOD AND APPARATUS FOR PERFORMING HAND-OFF BETWEEN SECTORS OF A COMMON BASE STATION,” filed Dec. 11, 1996, now U.S. Pat. No. 5,933,787, issued on Aug. 3, 1999, assigned to the present assignee. Thus, both soft and softer handoff for voice services result in redundant transmissions from two or more base stations to improve reliability.
This additional reliability is not so important for data traffic communications because the data packets received in error can be retransmitted. Important parameters for data services are transmission delay required to transfer a data packet and the average throughput rate of the data traffic communication system. The transmission delay does not have the same impact in data communication as in voice communication, but the transmission delay is an important metric for measuring the quality of the data communication system. The average throughput rate is a measure of the efficiency of the data transmission capability of the communication system. Because of relaxed transmission delay requirement, the transmit power and resources used to support soft handoff on the forward link can be used for transmission of additional data, thus, increasing average throughput rate by increasing efficiency.
The situation is different on the reverse link. Several base stations can receive the signal transmitted by a subscriber station. Because re-transmission of packets from a subscriber station requires additional power from a power limited source (a battery), it may be efficient to support soft handoff on the reverse link by allocating resources at several base stations to receive and process the data packets transmitted from the subscriber station. Such a utilization of soft handoff increases both coverage and reverse link capacity as discussed in a paper by Andrew J. Viterbi and Klein S. Gilhousen: “Soft Handoff Increases CDMA coverage and Increases Reverse Link Capacity,” IEEE Journal on Selected Areas in Communications, Vol. 12, No. 8, October 1994. The term soft hand-off is a communication between a subscriber station and two or more sectors, wherein each sector belongs to a different cell. In the context of the IS-95 standard, the reverse link communication is received by both sectors, and the forward link communication is simultaneously carried on the two or more sectors' forward links. In the context of the IS-856 standard, data transmission on the forward link is non-simultaneously carried out between one of the two or more sectors and the access terminal. Additionally, a softer handoff, may be used for this purpose. The term softer handoff is a communication between a subscriber station and two or more sectors, wherein each sector belongs to the same cell. In the context of the IS-95 standard, the reverse link communication is received by both sectors, and the forward link communication is simultaneously carried on one of the two or more sectors' forward links. In the context of the IS-856 standard, data transmission on the forward link is non-simultaneously carried out between one of the two or more sectors and the access terminal.
It is well known that quality and effectiveness of data transfer in a wireless communication system is dependent on the condition of a communication channel between a source terminal and a destination terminal. Such a condition, expressed as, for example, a signal-to-interference-and-noise-ratio (SINR), is affected by several factors, e.g., a path loss and the path loss' variation of a subscriber station within a coverage area of a base station, interference from other subscriber stations both from the same-cell and from other-cell, interference from other base stations, and other factors known to one of ordinary skill in the art. In order to maintain a certain level of service under variable conditions of the communication channel, TDMA and FDMA systems resort to separating users by different frequencies and and/or time-slots and support frequency reuse to mitigate the interference. Frequency reuse divides an available spectrum into many sets of frequencies. A given cell uses frequencies from only one set; the cells immediately adjacent to this cell may not use a frequency from the same set. In a CDMA system, the identical frequency is reused in every cell of the communication system, thereby improving the overall efficiency. The interference is mitigated by other techniques, e.g., orthogonal coding, transmission power control, variable rate data, and other techniques known to one of ordinary skill in the art.
The above-mentioned concepts were utilized in a development of a data traffic only communication system known as the High Data Rate (HDR) communication system. Such a communication system is disclosed in detail in co-pending application Ser. No. 08/963,386, entitled “METHOD AND APPARATUS FOR HIGH RATE PACKET DATA TRANSMISSION,” filed Nov. 3, 1997, now U.S. Pat. No. 6,574,211, issued on Jun. 3, 2003, assigned to the present assignee. The HDR communication system was standardized as a TIA/EIA/IS-856 industry standard hereinafter referred to as the IS-856 standard.
The IS-856 standard defines a set of data rates, ranging from 38.4 kbps to 2.4 Mbps, at which an access point (AP) may send data to a subscriber station (access terminal). Because the access point is analogous to a base station, the terminology with respect to cells and sectors is the same as with respect to voice systems. In accordance with the IS-856 standard, the data to be transmitted over the forward link are partitioned into data packets, with each data packet being transmitted over one or more intervals (time-slots), into which the forward link is divided. At each time-slot, data transmission occurs from an access point to one and only one access terminal, located within the coverage area of the access point, at the maximum data rate that can be supported by the forward link and the communication system. The access terminal is selected in accordance with forward link conditions between the access point and an access terminal. The forward link conditions depend on interference and path loss between an access point and an access terminal, both of which are time-variant. The path loss and the variation of the path loss are exploited by scheduling the access point's transmissions at time intervals, during which the access terminal's forward link conditions to a particular access point satisfy determined criteria that allow for transmissions with less power or higher rate of data than transmissions to the remaining access terminals, thus improving spectral efficiency of forward link transmissions.
In contrast, according to the IS-856 standard, data transmissions on the reverse link occur from multiple access terminals located within a coverage area of an access point. Furthermore, because the access terminals' antenna patterns are omni-directional, any access terminal within the coverage area of the access point may receive these data transmissions. Consequently, the reverse link transmissions are subjected to several sources of interference: code-division multiplexed overhead channels of other access terminals, data transmissions from access terminals located in the coverage area of the access point (same-cell access terminals), and data transmissions from access terminals located in the coverage area of other access points (other-cell access terminals).
With the development of wireless data services, the emphasis has been on increasing data throughput on the forward link, following the model of Internet services; where a server provides high rate data in response to requests from a host. The server-to-host direction is akin to a forward link requiring a high throughput, while the host-to-server requests and/or data transfers are at lower throughput. However, present developments indicate a growth of reverse link data intense applications, e.g., file transfer protocol (FTP), video conferencing, gaming, constant bit rate services, and the like. Such applications require improved efficiency of the reverse link to achieve higher data rates, so that applications demanding high throughput over reverse link. Therefore, there is a need in the art to increase data throughput on the reverse link, ideally to provide symmetric forward and reverse links throughputs. The increased data throughput on the reverse link further creates need in the art for method and apparatus for a power control and a rate of data determination.
The above and further features of the invention are set forth with particularity in the appended claims and together with advantages thereof will become clearer form considerations of the following detailed description of embodiments of the invention given by way of example with reference to the accompanying drawings.